Guided migration of axons is an important conserved feature of neuronal development. Precise regulation of cytoskeletal dynamics is critical for proper axonal motility and guidance. Many known signaling proteins contribute to cytoskeletal remodeling, but it is not yet understood how their activities are coordinated during turning responses. Aplysia neuronal growth cones turn towards substrate-coated beads, requiring actin polymerization and signaling by Src. Preliminary data indicates that Src and its substrate, the Rho GTPase inhibitor p190RhoGAP colocalize to the bead target site along with Rac GTPase early in the response. Potentially, Src/GTPase signaling coordinates a biphasic switch during turning responses from a polymerizing/noncontractile early phase characterized by high Rac activity / low Rho activity, to a high Rac / high Rho contractile phase. To test this hypothesis, I propose to 1) establish whether Rho and Rac GTPase activities are spatially coordinated into segregated domains using GFP-conjugated biosensors and a newly designed solvent-sensitive Rho biosensor and 2) correlate temporal changes in the spatial distributions of GTPase activities with cytoskeletal dynamic remodeling as visualized with TIRF-FSM. [unreadable] [unreadable]